296 McNair—A Method in Teaching Optical Mineralogy. 



been pointed but, but which are sufficiently close to avoid the 

 possibility of error in any practical application. 1 have used 

 this device with many students, and its justification rests in 

 the readiness with which a student who once comprehends the 

 meaning of the ellipsoid becomes independent in his applica- 

 tion of this test of the quartz wedge, applying the wedge in 

 either position, and reasoning out his results with an assurance 

 of correctness. If there were no other application I should 

 consider that the results have justified my continued use of 

 the scheme. It applies of course to the uniaxial crystal and 

 the quarter wave-plate, or the first order red, as readily as to 

 the biaxial case. 



Michigan College of Mines, Houghton, Michigan, i 



Jan. 14th, 1911. 



